Really Tough Target Sh-2-72, Sh-2-71, NGC6749

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midwayexpress
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Really Tough Target Sh-2-72, Sh-2-71, NGC6749

Post by midwayexpress »

This is one of the only objects I can think of where you can get a planetary nebula, emission nebula, and globular cluster in your field of view. The bad news is that Sh-2-72 is sooo dim that 2.5 hours of exposure without a HA filter revealed absolutely nothing. Not to be differed I went and purchased a Future Optics 12nm Ha filter and went to work. I currently use a color Atik 490 so figuring out how to process this image was a bit of a challenge. One thing I decided right off the bat is that I would not try and keep full resolution and instead opt to split the channels. I may have tried full resolution but CCDStack doesn't support drizzle.

Method 1) Ha as Luminance. I stacked each channel separately, then processed, saved, and recombined in StarTools using my Ha master as luminance.
http://astrob.in/193516/A/

Method 2) Synth Luminance. I followed the process described here http://forum.startools.org/viewtopic.php?f=7&t=236
http://astrob.in/193516/B/

Method 3) Synth Luminance. I followed the process described here http://forum.startools.org/viewtopic.php?f=7&t=236 but then used the RGB + Lum combine method here http://forum.startools.org/viewtopic.ph ... 2532#p2532
http://astrob.in/193516/C/

I also tried the NBRGBCombination and HaRVG-AIP scripts in PixInsight and frankly had no luck. If anyone has used either method I would love to see your work flow. The HaRVG-AIP method in particular did something that produced a FIT file that was only open able in PixInsight. Also neither one of these methods correctly accounts for the Hb inside of the Ha channel whereas the methods I used do.
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Re: Really Tough Target Sh-2-72, Sh-2-71, NGC6749

Post by admin »

Would you be able to post the data. so we can have a try?
Ivo Jager
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midwayexpress
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Re: Really Tough Target Sh-2-72, Sh-2-71, NGC6749

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Absolutely, I actually added 1.5 hours more of data last night for 7 hours total. Not a whole lot better but it helps on such a faint target. The zip fie below has a stack of each channel plus the stack of the ha channel.

http://1drv.ms/1f1ycH8

Edit: I made 3 more processing attempts incorporating the new data and ended up beating my head on the Synth Lum method for some reason.

This one uses the HARVB-AIP script in PixInsight. I processed both the HA Luminance and the RGB combo file in StarTools first. This appears to have the best colors of the group.
http://www.astrobin.com/193516/D/

This one uses the Synth Lum + LRGB combine in StarTools method. I preprocessed both files. The problem with this particular image is that I can't seem to get the red color in it like I did with the previous synthlum attempts. I have no idea why.
http://www.astrobin.com/193516/E/

This one uses the Synth Lum + Layer Extract Color StarTools method. I preprocessed both files. This guy has better nebula color but compared to the HARVB and the files I did last night the red is still week and the globular cluster stars are too red.
http://www.astrobin.com/193516/F/
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Re: Really Tough Target Sh-2-72, Sh-2-71, NGC6749

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Thanks for sharing that data! It appears Ha and RGB aren't aligned, while the RGB data shows some severe gradients. Did you take flats at all? Especially when imaging dim objects, they are not optional!
As you've shown, there is definitely signal in the Ha stack, but there is very little recoverable signal in the RGB data to augment the Ha as luminance.

Given the data, I think you've done a reasonable job!
Ivo Jager
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midwayexpress
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Re: Really Tough Target Sh-2-72, Sh-2-71, NGC6749

Post by midwayexpress »

I'm sorry I forgot to give the crop numbers to align the ha to rgb or I accidentally didn't attach the registered ha file. I will check when I'm on my computer.

I definitely took flats. The challenge is the light pollution I'm shooting from. I'm in a red/white zone. I do use a idas lpf.

Edit: any idea why the color came through on my first attempt but not on my second attempt?
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Re: Really Tough Target Sh-2-72, Sh-2-71, NGC6749

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midwayexpress wrote:I'm sorry I forgot to give the crop numbers to align the ha to rgb or I accidentally didn't attach the registered ha file. I will check when I'm on my computer.
Thanks! :thumbsup:
midwayexpress wrote:I definitely took flats. The challenge is the light pollution I'm shooting from. I'm in a red/white zone. I do use a idas lpf.
Hmmmm, yes... That is a challenge. Reason I asked was that I saw a typical fall off towards the corners in the RGB data. Note that the LP filter will skew your colors as well, particularly impacting yellow in your images.
midwayexpress wrote:Edit: any idea why the color came through on my first attempt but not on my second attempt?
The color recovery methods are subtly different between LRGB (uses L to weigh RGB ratios) and the color inheritance/layering technique (uses CIELab color space). With very low signal this can definitely account for the discrepancy... :think:
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midwayexpress
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Re: Really Tough Target Sh-2-72, Sh-2-71, NGC6749

Post by midwayexpress »

I had not included the registered HA file in the original zip file. You can download that here.
http://1drv.ms/1V848uu

I used the LRVB script last night to combine the synth lum and the RGB files I processed in PixInsight. Here is the result which is not bad but the contrast between the yellow in the globular and red in the nebula is not good compared to just the HA version.
http://www.astrobin.com/193516/G/

Here are the crop factors I used on all images
16,1204
1567,16
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